Biogenicity of Earth's earliest fossils: A resolution of the controversy

The abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the ~ 3465 Ma Apex chert of northwestern Australia — widely regarded as among the oldest records of life — have been investigated intensively. First reported in 1987 and formally described in 1992 and 1993, the biogenicity of the Apex fossils was questioned in 2002 and in three subsequent reports. However, as is shown here by use of analytical techniques unavailable twenty years ago, the Apex filaments are now established to be bona fide fossil microbes composed of three-dimensionally cylindrical organic- (kerogenous-) walled cells. Backed by a large body of supporting evidence of similar age — other microfossils, stromatolites, and carbon isotopic data — it seems clear that microbial life was present and flourishing on the early Earth ~ 3500 Ma ago.     

The Paleozoic evolution of Central Tianshan: Geochemical and geochronological evidence

A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~ 450–400 Ma, ~ 370–350 Ma and ca. 340 Ma. The first stage (450–400 Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)_CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350 Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~ 365 Ma) granitoids fit within the A-type field whereas the younger (~ 352 Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340 Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf T_DM^C model age populations: ca. 2400 Ma, ca. 1400 Ma and ca. 1100 Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~ 2400 Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~ 1400 Ma) and the assembly of Rodinia (~ 1100 Ma).     

Late Miocene–Quaternary rapid stepwise uplift of the NE Tibetan Plateau and its effects on climatic and environmental changes

The way in which the NE Tibetan Plateau uplifted and its impact on climatic change are crucial to understanding the evolution of the Tibetan Plateau and the development of the present geomorphology and climate of Central and East Asia. This paper is not a comprehensive review of current thinking but instead synthesises our past decades of work together with a number of new findings. The dating of Late Cenozoic basin sediments and the tectonic geomorphology of the NE Tibetan Plateau demonstrates that the rapid persistent rise of this plateau began ~ 8 ± 1 Ma followed by stepwise accelerated rise at ~ 3.6 Ma, 2.6 Ma, 1.8–1.7 Ma, 1.2–0.6 Ma and 0.15 Ma. The Yellow River basin developed at ~ 1.7 Ma and evolved to its present pattern through stepwise backward-expansion toward its source area in response to the stepwise uplift of the plateau. High-resolution multi-climatic proxy records from the basins and terrace sediments indicate a persistent stepwise accelerated enhancement of the East Asian winter monsoon and drying of the Asian interior coupled with the episodic tectonic uplift since ~ 8 Ma and later also with the global cooling since ~ 3.2 Ma, suggesting a major role for tectonic forcing of the cooling.     

Geochemistry,zircon UPb geochronology and LuHf isotopic composition of eclogites and their host gneisses in the Dulan area,North Qaidam UHP terrane: New evidence for deep continental subduction

In this study, we link zircon UPb SHRIMP and LA-ICP-MS geochronology and the LuHf isotopic composition of eclogites and their host gneisses/schists with whole-rock geochemistry of eclogites in the Dulan area to constrain their protoliths and metamorphic relationships. UPb dating suggests that the protolith of one of the eclogites was a Neoproterozoic mafic intrusive rock (828 ± 58 Ma) and the protolith of enclosing orthogneiss was an early-Neoproterozoic granitoid (923 ± 12 Ma). Detrital zircons from Grt-bearing mica-schists yield ages of 0.9–2.5 Ga, with a dominant range of 1.0–1.8 Ga, indicating sedimentary sources from Neoproterozoic to Neoarchean crust and a depositional age ≤ 0.9 Ga. The matching metamorphic ages of eclogites (438 ± 5 Ma, 436 ± 4 Ma) and their country rocks (Grt-bearing mica-schists: 438 ± 4 Ma, 439 ± 8 Ma; orthogneiss: 427 ± 8 Ma) indicate that all studied samples experienced coeval Early Paleozoic HP/UHP metamorphism. The UPb ages and Hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample (ε_Hf (800) = 2.6–9.2, T_DM1 = 1.0–1.3 Ga, T_DM2 = 1.1–1.4 Ga) suggest that the protolith may be derived from Neoproterozoic depleted mantle with variable proportions of an older crustal component. The magmatic zircon cores of the orthogneiss (ε_Hf (900) = ? 7.3 to ? 0.2; T_DM2 = 1.8–2.1 Ga) suggest that the parental magma was derived from a Paleoproterozoic crustal source. Hf isotopic compositions of the detrital zircons from the metasediments (ε_Hf(t) = ? 19.4 to + 10.6) suggest three crust formation and reworking events: (1) Archean (T_DM2 = 2.7–2.9 Ga) juvenile crust reworked at ~ 2.5 Ga; (2) early Paleoproterozoic (T_DM2 = 2.3–2.5 Ga) juvenile crust reworked at ~ 1.8 Ga; and (3) late Paleoproterozoic (T_DM2 = 1.5–1.9 Ga) juvenile crust reworked in the Neoproterozoic.Whole-rock geochemical data suggest that the protoliths of the Dulan eclogites were probably derived from a continental rift or an incipient oceanic basin rather than a large, long-lived ocean basin. Thus, combined with field relationships, petrology, geochemistry, zircon UPb dating and the LuHf isotopic analysis presented in this paper and reported from previous studies, we suggest that the Dulan eclogites and their country rocks experienced a common UHP metamorphism during Late Ordovician deep continental subduction.     

Petrogenesis of the Guangtoushan granitoid suite,central China: Implications for Early Mesozoic geodynamic evolution of the Qinling Orogenic Belt

The Qinling Orogenic Belt marks the link between the South China and North China Blocks and is an important region to understand the geological evolution of the Chinese mainland as well as the Asian tectonic collage. However, the tectonic affinity and geodynamic evolution of the South Qinling Tectonic Belt (SQTB), a main unit of the Qinling Orogenic Belt, remains debated. Here we present detailed geological, geochemical and zircon U–Pb–Hf isotopic studies on the Zhangjiaba, Xinyuan, Jiangjiaping, Guangtoushan and Huoshaodian plutons from the Guangtoushan granitoid suite (GGS) in the western segment of the SQTB. Combining geology, geochronology and whole-rock geochemistry, we identify four distinct episodes of magmatism as: (1) ~ 230–228 Ma quartz diorites and granodiorites, (2) ~ 224 Ma fine-grained granodiorites and monzogranites, (3) ~ 218 Ma porphyritic monzogranites and (4) ~ 215 Ma high-Mg# quartz diorites and granodiorites as well as coeval muscovite monzogranites. The ~ 230–228 Ma quartz diorites and granodiorites were generated by magma mixing between a mafic melt from mantle source and a granodioritic melt derived from partial melting of Neoproterozoic rocks in the lower continental crust related to a continental arc regime. The ~ 224 Ma fine-grained granodiorites and monzogranites were formed through partial melting of a transitional source with interlayers of basaltic rocks and greywackes in the deep zones of the continental arc. The ~ 218 Ma porphyritic monzogranites originated from partial melting of metamorphosed greywackes in lower crustal levels, suggesting underthrusting of middle or upper crustal materials into lower crustal depths. The ~ 215 Ma high-Mg# quartz diorites and granodiorites (with Mg# values higher than 60) were derived from an enriched mantle altered by sediment-derived melts. Injection of hot mantle-derived magmas led to the emergence of the ~ 215 Ma S-type granites at the final stage.Integrating our studies with previous data, we propose that the Mianlue oceanic crust was still subducting beneath the SQTB during ~ 248–224 Ma, and final closure of the Mianlue oceanic basin occurred between ~ 223 Ma and ~ 218 Ma. After continental collision between the South China Block and the SQTB, slab break-off occurred, following which the SQTB transformed into post-collisional extension setting.     

Crustally-derived granites in the Panzhihua region,SW China: Implications for felsic magmatism in the Emeishan large igneous province

In the Panxi region of the Late Permian (~ 260 Ma) Emeishan large igneous province (ELIP) there is a bimodal assemblage of mafic and felsic plutonic rocks. Most Emeishan granitic rocks were derived by differentiation of basaltic magmas (i.e. mantle-derived) or by mixing between crustal melts and primary basaltic magmas (i.e. hybrid). The Yingpanliangzi granitic pluton within the city of Panzhihua intrudes Sinian (~ 600 Ma) marbles and is unlike the mantle-derived or hybrid granitic rocks. The SHRIMP zircon U–Pb ages of the Yingpanliangzi pluton range from 259 ± 8 Ma to 882 ± 22 Ma. Younger ages are found on the zircon rims whereas older ages are found within the cores. Field relationships and petrography indicate that the Yingpanliangzi pluton must be < 600 Ma, therefore the older zircons are interpreted to represent the protolith age whereas the younger analyses represent zircon re-crystallization during emplacement. The Yingpanliangzi granites are metaluminous and have negative Ta–Nb_PM anomalies, low εNd_{(260 Ma)} values (? 3.9 to ? 4.4), and high I_Sr (0.71074 to 0.71507) consistent with a crustal origin. The recognition of a crustally-derived pluton along with mantle-derived and mantle–crust hybrid plutons within the Panxi region of the ELIP is evidence for a complete spectrum of sources. As a consequence, the types of Panxi granitoids can be distinguished according to their ASI, Eu/Eu*, εNd_(T), εHf_(T), T_Zr(°C) and Nb–Ta_PM values. The diverse granitic magmatism during the evolution of the ELIP from ~ 260 Ma to ~ 252 Ma demonstrates the complexity of crustal growth associated with LIPs.     

U–Pb and Hf isotope data from zircons in the Macquarie Arc,Lachlan Orogen: Implications for arc evolution and Ordovician palaeogeography along part of the east Gondwana margin

The Ordovician Macquarie Arc in the eastern subprovince of the Lachlan Orogen, southeastern Australia, is an unusual arc that evolved in four vertically stacked volcanic phases over ~ 37 million years, and which is flanked by coeval, craton-derived, passive margin sedimentary terranes dominated by detrital quartz grains. Although these two terranes are marked by a general absence of provenance mixing, LA-ICPMS analysis of U–Pb and Lu–Hf contents in zircon grains in volcaniclastic rocks from 3 phases of the arc demonstrates the same age populations of detrital grains inherited from the Gondwana margin as those that characterise the flanking quartz-rich Ordovician turbidites. Magmatic Phase 1 is older, ~ 480 Ma, and is characterised by detrital zircons grains with ages of ~ 490–540 with negative ε_Hf from 0 to mainly –7.78, 550–625 Ma ages with negative ε_Hf from 0 to ?26.6 and 970–1250 Ma (Grenvillian) with ε_Hf from + 6.47 to ?6.44. We have not as yet identified any magmatic zircons related to Phase 1 volcanism. Small amounts of detrital zircons also occur in Phase 2 (~ 468–455 Ma), hiatus 1 and Phase 4 (~ 449–443 Ma), all of which are dominated by Ordovician magmatic zircons with positive ε_Hf values, indicating derivation from unevolved mantle-derived magmas, consistent with formation in an intraoceanic island arc. Because of the previously obtained positive whole rock ε_Nd values from Phase 1 lavas, we rule out contamination from substrate or subducted sediments. Instead, we suggest that during Phase 1, the Macquarie Arc lay close enough to the Gondwana margin so that volcaniclastic rocks were heavily contaminated by detrital zircon grains shed from granites and Grenvillian mafic rocks mainly from Antarctica (Ross Orogen and East Antarctica) and/or the Delamerian margin of Australia. The reduced nature of a Gondwana population in Phase 2, hiatus 1 and Phase 4 is attributed to opening of a marginal basin between the Gondwana margin and the Macquarie Arc that put it out of reach of all but rare turbiditic currents.     

Early Mesozoic southward subduction history of the Mongol–Okhotsk oceanic plate: Evidence from geochronology and geochemistry of Early Mesozoic intrusive rocks in the Erguna Massif,NE China

In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits.     

A possible transition from island arc to continental arc magmatism in the eastern Jiangnan Orogen,South China: Insights from a Neoproterozoic (870–860 Ma) gabbroic–dioritic complex near the Fuchuan ophiolite

The Fuchuan ophiolite belt in the eastern Jiangnan Orogen of South China provides important constraints on the tectonic setting and evolution of the Neoproterozoic suture zone between the Yangtze and Cathaysia blocks. Combined UPbHf isotopic and REE analysis of zircon from gabbroic and dioritic samples of the Shexian complex, located 10 km southwest of the main Fuchuan ophiolite body, indicate that the complex crystallized at ca. 870–860 Ma with a large variation of zircon ε_Hf(t) values from − 4.80 to + 13.30. Whole-rock geochemistry reveals that the magma mainly experienced fractionation of olivine, clinopyroxene and plagioclase and was partly affected by crustal contamination, which resulted in elevated Th/Nb, Th/La and La/Sm ratios, as well as the scattered ε_Hf(t) values. The most mafic and least contaminated sample shows MORB affinity and was probably formed by partial melting of a depleted subduction-metasomatized mantle wedge. Other samples exhibit arc-like signatures and were probably modified by both melt- and fluid-related subduction metasomatism. The emplacement of the Shexian complex corresponds to the time that subduction switched from a ca. 1000–880 Ma intra-oceanic island arc to a 870–830 Ma continental arc along the southeastern Yangtze Block. The sequence of igneous rocks associated with this continental arc resemble those preserved in forearc Tethyan ophiolites, with magma evolving from ca. 870–860 Ma MORB to ca. 860–850 Ma arc tholeiite and ca. 830 Ma boninite. Arc magmatism concluded with the final assembly of the Yangtze and Cathaysia blocks at 830–800 Ma.     

Depositional age,provenance and metamorphic age of metasedimentary rocks from southern Madagascar

Southern Madagascar is the core of a > 1 million km² Gondwanan metasedimentary belt that forms much of the southern East African Orogen of eastern Africa, Madagascar, southern India and Sri Lanka. Here the Vohibory Series yielded U–Pb isotopic data from detrital zircon cores that indicate that it was deposited in the latest Tonian to late Cryogenian (between ~ 900 and 640 Ma). The deposition of the Graphite and Androyen Series protoliths is poorly constrained to between the late Palaeoproterozoic and the Cambrian (~ 1830–530 Ma). The Vohibory Series protoliths were sourced from very restricted-aged sources with a maximum age range between 910 and 760 Ma. The Androyen and Graphite Series protoliths were sourced from Palaeoproterozoic rocks ranging in age between 2300 and 1800 Ma. The best evidence of the timing of metamorphism in the Vohibory Series is a weighted mean ²⁰⁶Pb/²³⁸U age of 642 ± 8 Ma from 3 analyses of zircon from sample M03-01. A considerably younger ²⁰⁶Pb/²³⁸U metamorphic age of 531 ± 7 Ma is produced from 10 analyses of zircon from sample M03-28 in the Androyen Series. This ~ 110 Ma difference in age is correlated with the early East African Orogeny affecting the west of Madagascar along with its type area in East Africa, whereas the Cambrian Malagasy Orogeny affected the east of Madagascar and southern India during the final suturing of the Mozambique Ocean.     

Palaeozoic to Early Jurassic history of the northwestern corner of Gondwana,and implications for the evolution of the Iapetus,Rheic and Pacific Oceans

The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic.     

Geochronologic (ReOs) and fluid-chemical constraints on the formation of the Mesoproterozoic-hosted Nanisivik ZnPb deposit,Nunavut, Canada: Evidence for early diagenetic,low-temperature conditions of formation

The age and origin of the past-producing Nanisivik carbonate-hosted ZnPb deposit in Nunavut, Canada, have been controversial for decades. Various direct and indirect dating methods have produced results ranging from Mesoproterozoic to Ordovician in age, and previous studies of the mineralising fluids have suggested that the fluids were anomalously hot (> 150 °C). This study combines ReOs (pyrite) geochronology, in-situ sulphur isotope analysis, and fluid inclusion analysis to refine both the timing of mineralisation and the nature of mineralising fluids. ReOs pyrite analysis shows that the Nanisivik deposit formed ca. 1.1 Ga, broadly similar to the depositional age of the host rock and with the Grenville orogeny, making it one of few known Precambrian carbonate-hosted ZnPb deposits. In-situ sulphur isotope measurements from Nanisivik show a narrow δ³⁴S range of 27.54 ± 0.72, very similar to what has been reported before in bulk sample analyses. New fluid inclusion data show that the mineralising fluids were ~ 100 °C, which is not anomalous in the context of carbonate-hosted base-metal deposits. The fluids exhibit no significant spatial variation in homogenisation temperature in the 2-km-long ‘upper lens’ of the ore deposit, but recrystallisation and modification of fluid inclusions took place in the immediate vicinity of the cross-cutting ~ 720 Ma “mine dyke”. The deposit is broadly inferred to have formed during late Mesoproterozoic assembly of supercontinent Rodinia, when regional hydrostatic head developed under the influence of far-field stresses originating in the developing Grenville orogen. The Nanisivik deposit remains anomalous only in its age; most other aspects of this ore deposit are now shown to be quite typical for carbonate-hosted ore deposits.     

FTIR microspectroscopy of Ediacaran phosphatized microfossils from the Doushantuo Formation,Weng'an,South China

Phosphatized microfossils from ca. 580 Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960 cm^− 1 and 2925 cm^− 1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH₃ and aliphatic CH₂), and have an additional band of around 1595 cm^− 1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH₃/CH₂ peak height ratios (R_3/2) and aromatic CC/CH₂ peak height ratios (R_CC/2 values): (1) high-R_3/2/low-R_CC/2 type (R_3/2 = ~ 0.2–1.0 and R_CC/2 ~ 0–2), (2) low-R_3/2/medium-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 = ~ 1–4); and (3) low-R_3/2/high-R_CC/2 type (R_3/2 = ~ 0.2–0.6 and R_CC/2 ~ 1–8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R_3/2 and R_CC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria.     

Combined paleomagnetic and geochronological study on Cretaceous strata of the Qiangtang terrane,central Tibet

A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8 ± 0.46 Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D = 348.0°, I = 47.3°, k = 51.0, α₉₅ = 5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A₉₅ = 5.7° and yielding a paleolatitude of 29.3° ± 5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~ 100 Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D = 312.6°, I = 47.7°, k = 109.7, α₉₅ = 3.0°, N = 22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A₉₅ = 3.2° (yielding a paleolatitude of 28.7° ± 3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~ 100 Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~ 550 km N–S convergence between the Qiangtang and Lhasa terranes happened after ~ 100 Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~ 100 Ma, which is most likely caused by the India–Asia collision.     

U–Pb and Re–Os isotopic systematics and zircon Ce4 +/Ce3 + ratios in the Shiyaogou Mo deposit in eastern Qinling,central China: Insights into the oxidation state of granitoids and Mo (Au) mineralization

The newly-discovered Shiyaogou molybdenum deposit is located in the eastern Qinling metallogenic belt in central China. The deposit contains at least 152,000 t of Mo metal and bears typical porphyry-type features in terms of its concentric alteration zonation, quartz vein-hosted Mo mineralization, veining sequence and the spatial association with concealed granite porphyries. Re–Os isotope analyses of molybdenite from the deposit yield an ore-forming age of 132.3 ± 2.8 Ma. LA-ICP-MS U–Pb zircon dating of ore-related porphyries yields crystallization ages from 135 Ma to 132 Ma, indicating a temporal link between granitic magmatism and Mo mineralization. A population of captured magmatic zircons indicates another pulse of magmatism at ~ 143 Ma. A barren granite intrusion near the deposit gives a zircon U–Pb age of 148.1 ± 1.1 Ma. These magmatic activities were concurrent with the emplacement of the nearby Heyu granitic batholith, a largely ore-barren intrusive complex formed from ~ 148 Ma to ~ 127 Ma. Zircon Ce^4 +/Ce^3 + ratios of ore-related porphyries are obviously higher than those of contemporaneous barren granitoids, implying an affinity between Mo mineralization and highly oxidized magmas. Moreover, zircons from these granitoids overall have decreasing Ce^4 +/Ce^3 + ratios from 148 Ma to 132 Ma, reflecting decreasing oxygen fugacities during magma evolution. Available geological, radiometric and stable isotopic evidence suggests that the decrease of magma oxygen fugacity was probably associated with an increase of mantle contribution to granitic magmatism and metallogenesis, which probably gave rise to successive mineralization of Mo and Au in the eastern Qinling. The intense magmatic–metallogenic events in the eastern Qinling during Late Jurassic to Early Cretaceous times are interpreted as a response to the large-scale lithosphere thinning and subsequent asthenosphere upwelling beneath the eastern part of the North China Craton.     

Postdated melting of subcontinental lithospheric mantle by the Emeishan mantle plume: Evidence from the Anyi intrusion,Yunnan, SW China

The Anyi intrusion is located in the central zone of Emeishan large igneous province (ELIP), SW China. It outcrops in an area of about 0.65 km² and ~ 1 km thick and dips to the southwest. The Anyi intrusion consists of a lower clinopyroxenite zone, middle gabbro zone, and an upper monzonite–syenite zone. Up to 400 m thick stratiform disseminated Fe–Ti oxide layer with grades of 16–18 wt.% total Fe is hosted in the lower clinopyroxenite zone. Zircon SHRIMP U–Pb age (247 ± 3 Ma) indicates that the Anyi intrusion represents postdated mafic magmatism resulting from the ~ 260 Ma Emeishan mantle plume. Compared with the typical oxide-bearing intrusions (such as Panzhihua and Baima) formed at ~ 260 Ma in the ELIP, the Anyi intrusion is characterized by high alkaline contents and LREE/HREE ratios, extremely low εNd values (− 6.2 to − 7.6) and moderate high (⁸⁷Sr/⁸⁶Sr)_i values (0.7072 to 0.7086). These characteristics of the Anyi intrusion cannot be explained by fractional crystallization or crustal contamination, but may reflect a unique enriched continental lithospheric mantle source (a mantle source mixed between garnet pyroxenite and spinel peridotite). We propose that the postdated mafic magmatism associated with the formation of the Anyi intrusion and its Fe–Ti oxide ore may be the product of melting of a mantle source mixed between garnet pyroxenite and spinel peridotite in the shallow lithosphere caused by conductive heating combined with lithosphere thinning due to plume–lithosphere interaction.     

Tectonics of the North Qilian orogen,NW China

The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥ 710 Ma as a consequence of breakup of supercontinent Rodinia.Initiation of the subduction in the Qilian Ocean probably occurred at ~ 520 Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~ 517–490 Ma. In the beginning of Ordovician (~ 490 Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of ~ 490–445 Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~ 6–7 °C/km.The Qilian Ocean was closed at the end of the Ordovician (~ 445 Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~ 100–200 km at about 435–420 Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials.Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation.     

Sedimentological and geochronological constraints on the Carboniferous evolution of central Inner Mongolia,southeastern Central Asian Orogenic Belt: Inland sea deposition in a post-orogenic setting

Sedimentological and geochronological analyses were performed on Carboniferous strata from central Inner Mongolia (China) to determine the tectonic setting of the southeastern Central Asian Orogenic Belt (CAOB). Sedimentological analyses indicate that the widespread Late Carboniferous strata in central Inner Mongolia were dominated by shallow marine clastic-carbonate deposition with basal conglomerate above the Precambrian basement and Early Paleozoic orogenic belts. Based on lithological comparison and fossil similarity, five sedimentary stages were used to represent the Carboniferous deposition. The depositional stages include, from bottom to top, 1) basal molassic, 2) first carbonate platform, 3) terrigenous with coeval intraplate volcanism, 4) second carbonate platform, and 5) post-carbonate terrigenous. These five stages provide evidence for an extensive transgression in central Inner Mongolia during the Late Carboniferous. Detrital zircon geochronological studies from five samples yielded five main age populations: ~ 310 Ma, ~ 350 Ma, 400–450 Ma, 800–1200 Ma and some Meso-Proterozoic to Neoarchean grains. The detrital zircon geochronological studies indicate that the provenances for these Late Carboniferous strata were mainly local magmatic rocks (Early Paleozoic arc magmatic rocks and Carboniferous intrusions) with subordinate input of Precambrian basement. Combining our sedimentological and provenance analyses with previous fossil comparison and paleomagnetic reconstruction, an inland sea was perceived to be the main paleogeographic feature for central Inner Mongolia during the Late Carboniferous. The inland sea developed on a welded continent after the collision between North China Craton and its northern blocks.     

Abrupt spatial and geochemical changes in lamprophyre magmatism related to Gondwana fragmentation prior,during and after opening of the Tasman Sea

High-precision ⁴⁰Ar/³⁹Ar dating of lamprophyre dike swarms in the Western Province of New Zealand reveals that these dikes were emplaced into continental crust prior to, during and after opening of the Tasman Sea between Australia and New Zealand. Dike ages form distinct clusters concentrated in different areas. The oldest magmatism, 102–100 Ma, is concentrated in the South Westland region that represents the furthest inboard portion of New Zealand in a Gondwana setting. A later pulse of magmatism from ~ 92 Ma to ~ 84 Ma, concentrated in North Westland, ended when the first oceanic crust formed at the inception of opening of the Tasman Sea. Magmatic quiescence followed until ~ 72–68 Ma, when another swarm of dikes was emplaced. The composition of the dikes reveals a dramatic change in primary melt sources while continental extension and lithospheric thinning were ongoing. The 102–100 Ma South Westland dikes represent the last mafic calc-alkaline magmatism associated with a long-lived history of the area as Gondwana's active margin. The 92–84 Ma North and 72–68 Ma Central Westland dike swarms on the other hand have strongly alkaline compositions interpreted as melts from an intraplate source. These dikes represent the oldest Western Province representatives of alkaline magmatism in the greater New Zealand region that peaked in activity during the Cenozoic and has remained active up to the present day. Cretaceous alkaline dikes were emplaced parallel to predicted normal faults associated with dextral shear along the Alpine Fault. Furthermore, they temporally correspond to polyphase Cretaceous metamorphism of the once distal Alpine Schist. Dike emplacement and distal metamorphism could have been linked by a precursor to the Alpine Fault. Dike emplacement in the Western Province coupled to metamorphism of the Alpine Schist at 72–68 Ma indicates a period of possible reactivation of this proto Alpine Fault before it served as a zone of weakness during the opening of the oceanic Emerald Basin (at ~ 45 Ma) and eventually the formation of the present-day plate boundary (~ 25 Ma–recent).